This invention relates generally to video signal switching circuits and particularly to color video signal switching circuits that are used in color television receivers or video monitors for selecting, at a pixel rate, between two different input sources of video signals.
The practice of mixing video signals for display on a cathode ray tube (CRT) viewing screen has created a need for video switching circuits for selectively coupling the different video inputs to the CRT. It is, of course, essential that switching between the different video signals be accomplished without significant distortion or interference. While the prior art includes switching circuits that operate at a line scanning rate, such video switches are not capable of clean switching at a video or pixel rate. An example of such a switching circuit is shown in U.S. Pat. No. 3,702,898 issued 11/14/72. As those skilled in the art know, a pixel is the smallest video element that can be generated on the CRT screen and is used primarily in conjunction with outputs from computers and digital sources. What the prior art lacks is a simple high speed video switching circuit which will permit selective or mixed viewing of two different signal sources at a video or pixel rate without distortion or interference.
The present invention uses open collector connected switching devices for driving the different video signal transistors into and out of saturation. The invention eliminates the normal switching delay caused by minority carriers being cleared out of the base region when the transistor is operated from cutoff to its linear region. Since the transistors are never in cutoff, the transition between video signals is very rapid and without a delay between the signals.